1. Field of the Invention
The present invention relates generally to digital signal processing circuitry and, in particular, to a digital audio signal processing circuit which eliminates system clock jitter from digital audio systems such as a compact disk player stereo system and the like.
2. Description of the Prior Art
In a digital audio system, such as a compact disk player stereo system, the quality of audio reproduction is dependent upon the accuracy and time displacement of each digital word converted to an analog pulse signal. As is best illustrated by FIG. 1, the last component in a typical digital audio system is a digital to analog conversion circuit which produces an analog signal that corresponds in magnitude to a digital word provided to the digital to analog conversion circuit. A digital to analog conversion circuit, for example, with sixteen bit accuracy will reproduce sixteen bit audio information to an accuracy of less than one least significant bit having a value of 1/65,536 of the analog full scale. The digital audio information is generally derived from a storage medium such as tape, disc or the like and the accuracy of the information is ensured through error checking and correction techniques programmed into and performed within the digital signal processor shown in FIG. 1.
The time placement of digitally converted signals in a digital audio system is dependent upon a conversion clock signal. Although there is accurate data representation, the conversion clock generally has a timing error of plus or minus one nanosecond. The timing error present in the conversion signal is defined as jitter.
The major functional blocks of the digital audio system of FIG. 1 are contained within large scale integrated circuits. These integrated circuits generally exhibit poor signal to power supply isolation among the numerous sub-function circuits/digital signal processing circuits within the integrated circuits of FIG. 1. The poor signal to power supply isolation facilitates complex mutual cross modulation among the sub-function circuits of the integrated circuits of FIG. 1. Logic induced modulation is the term used to describe this distortion producing mechanism. Clock signal phase errors are a result of logic induced modulation. Due to logic induced modulation, audio data is reproduced with time displaced samples.
One technique of the prior art used to remove jitter from digital audio systems is to introduce a reclocking circuit into the system which reduces clock jitter. Typically, reclocking techniques use a phase lock loop circuit before the digital to analog converter conversion clock signal input. These prior art phase lock loop circuits generally reduce absolute jitter from approximately the ten nonosecond region down to the five hundred picosecond region. However, due to poor power supply and signal isolation of the phase lock loop circuit, new tones may be created which cause distortion even though jitter is reduced.
Therefore it is an object of the present invention to provide an improved digital audio system.
It is a further object of the present invention to provide a means for substantially reducing jitter from digital audio systems.
It is yet a further object of the invention to provide a digital audio system which has signal and power isolation thereby substantially eliminating logic induced modulation.
It is still a further object of the present invention to provide an isolated power supply for the digital to analog converter of a digital audio system and thereby reduce peak-to-peak jitter below audible limits.
Various other advantages and novel features will become apparent to those skilled in the art as a more detailed description of the present invention is set forth below.